1. Field of the Invention
The present invention relates to direct transmission of optical signals from an image through an optical fiber. More specifically, the present invention relates to a method and apparatus using a holographic storage medium for direct transmission of an optical image which compensates for distortions occurring during transmission of the image.
2. Description of the Related Art
The optical transmission of information in the prior art is achieved by transforming the digitalized signal from one or two dimensional source information into a corresponding light pulse train, and then inputting the light pulse train to an optical fiber by an optical encoding new method. The receiver receives the transmitted information through the process of transforming the transmitted light pulse into a desired form of signals using optical devices and other electronics. However, with this type of information transmission method it is impossible to transmit at high speed a large amount of data such as for HDTV (High Definition TV) or 3-dimensional images.
Therefore, a new communication method was introduced which directly transmits the light from an image through the optical fiber. According to this method, the transmission time can be minimized and the architecture of the transmission system can be simplified, since it is not necessary to transform the electrical signal into an optical signal at the input of the transmission system, or the optical signal received at the receiver back into an electrical signal. Additionally, the transmission of voluminous data is made easier in comparison with the conventional method, since the optical information is transmitted in parallel. Thus, the technique of directly transmitting the optical image through an optical fiber will play an important role in next-generation communications technologies.
However, commercialization of this communication method requires solving the problem of the distortion of the received image due to the distortion of the polarization and loss of the spatial properties of optical information such as images when directly transmitted through optical fibers. Optical fibers used as the transmission media exhibit distortion characteristics caused by scattering, reflection, and coupling of modes when the signal is propagated therethrough, as does translucent glass or plastic. To circumvent this problem, research has recently been conducted on spectral coding and image compensation methods using a phase conjugate wave. However, this research is still in the early stages and a commercially viable transmission method has not yet been developed.
Research conducted so far on conventional methods of direct image transmission has been based upon double pass image transmission in which an original image is reproduced by a double pass of the optical image. As such, this research cannot result in a commercially feasible direct transmission system.
In order to provide a commercially viable solution, a multi-channel system in which information from an image is converted into resolution elements which are transmitted through separate channels allocated for each resolution elememt was presented in "Three-dimensional pictorial transmission in optical fibers," Applied Physics Letter, Vol. 28, No. 2, pp 88-89, January 1976 and "On transmission and recovery of three-dimensional image information in optical waveguides," Journal of Optical Society of America, Vol. 66, No. 4, pp 301-306, April 1976 by A. Yariv. However, this method was considered to be impractical due to the interaction of modes (modal dispersion) within the optical fiber.
Another method in which resolution elements of an image are transmitted through respective channels using different carriers is discussed in "All-optical image transmission through a single-mode fiber," Optics Letters, Vol. 17, No. 8, pp 613-615, Apr. 15, 1992 by E. G. Paek, et al. and "Two-dimensional image transmission through a single optical fiber by wavelength-time multiplexing," Applied Optics, Vol. 22, No. 23, pp 3826-3832, December 1983 by A. M. Tai. In this method, light sources with different wavelengths, arranged in rows and columns, are used as carriers for resolution elements of an image. More specifically, the image is first divided into lines by sorting the point light sources in accordance with wavelengths to correspond to resolution elements of the image to be transmitted by means of a diffraction grating. After these lines are rearranged in some manner at the output, images are simultaneously transmitted line by line. However, a drawback of this method is the transmission delay caused by use of the mechanical elements needed to scan the image frame to be transmitted line by line. Theoretically, the real time transmission of the entire image frame can be achieved by utilizing a diffraction grating which can transmit an entire image frame at once instead of line by line. However, a method utilizing such a diffraction grating has not yet been developed.
Recently, a direct image transmission method using a 2D multicolor surface-emitting laser-diode array (MC SELDA) as carriers has been reported in "All-optical image transmission through a single-mode fiber," Optics Letters, Vol. 17, No. 8, pp 613-615, Apr. 15, 1992 by E. G. Paek et at. According to this method, the transmission of a complex image is made possible by using separate laser diodes of different wavelengths for each resolution element of the image to be transmitted. This method has also not been put to practical use since an array element with many laser diodes of different wavelengths is required.
Another method currently being studied in the field of direct transmission of images is an image compensation method using a phase conjugate wave in which an optical signal is passed through a first optical fiber and then through an ideal optical fiber similar in optical characteristics to the first optical fiber, as described in "Three-dimensional pictorial transmission in optical fibers," Applied Physics Letter, Vol. 28, No. 2, pp 88-89, January 1976 by A. Yariv. However, this is not a viable approach since it is impossible to make two optical fibers sufficiently similar to each other. Thus, research related to this method is directed mainly towards the application to a fiber optic gyroscopes, etc., in which the distortion is compensated for by looping the transmitted signal back through the same optical fiber that it first passed through. In addition to this, direct transmission of images using two consecutive waves shown in the publication, "Phase conjugation optical technologies," Advanced Optical Technology, SHOFU BOOKS, High-Technology Series, Tokyo, pp 202-235, 1988 by Fuji Yoichi and Nishijawa Koichi has not been achieved since a photorefractive crystal may not eliminate the distortion of the signal beam due to difficulties caused by the coupling of four waves in controlling image information of the signal beam by a reference beam.
Hence, a need exists in the art for an improved method of direct image transmission.